Gi/portal knowledge base (#6993)

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## PR-Codex overview
This PR primarily focuses on enhancing the `thirdweb Knowledge Base` by adding new troubleshooting guides, updating existing resources, and improving documentation structure for better user support.

### Detailed summary
- Added a new troubleshooting guide for `Nebula`.
- Introduced detailed steps for resolving `device link errors` in `thirdweb CLI`.
- Updated support links in `Header.tsx`.
- Created new sections for `batch upload issues` and `airdrop contract troubleshooting`.
- Improved layout in `layout.tsx` with `DocLayout`.
- Removed unused `ContractsIcon` from `page.tsx`.
- Enhanced the `knowledge base` sidebar structure.
- Added visual guides and images in multiple `mdx` files for clarity.
- Improved metadata for better SEO and documentation clarity.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Introduced a comprehensive Knowledge Base with structured navigation, guides, and troubleshooting for web3 development and thirdweb support.
  - Added detailed documentation on deploying NFT mint sites, creating soulbound tokens, sharing console errors, and resolving CLI device link errors.
  - Launched troubleshooting guides for airdrop contracts, batch upload issues, embed feature changes, and Nebula-specific support.

- **Improvements**
  - Updated navigation links to add a Nebula API reference and consolidated support links to internal resources.

- **Removals**
  - Removed the "Contracts" article card from the learning resources section.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: GiselleNessi

apps/portal/src/app/Header.tsx

@@ -122,6 +122,10 @@ export const connectLinks: Array<{
 ] as const;
 
 const apisLinks = [
+  {
+    name: "Nebula",
+    href: "/nebula/api-reference",
+  },
   {
     name: "Insight",
     href: "https://insight.thirdweb.com/reference",
@@ -139,11 +143,7 @@ const apisLinks = [
 const supportLinks = [
   {
     name: "Get thirdweb support",
-    href: "https://thirdweb.com/support",
-  },
-  {
-    name: "Knowledge Base",
-    href: "https://support.thirdweb.com",
+    href: "/knowledge-base",
   },
   {
     name: "Contact Sales",